Peracids electrophilic oxidants

We dealt in detail earlier in the chapter with reducing agents and their characteristic chemoselectivi-ties. Oxidizing agents are equally important, and in the chapter on electrophilic addition to alkenes we told you about peracids as oxidizing agents for C=C double bonds—they give epoxides. But... 

In heterocycles containing endocyclic sulfur, peracids are efficient electrophilic oxidizing reagents . One illustrative example is the preparation of 1,4,2-dithiazine 1,1,-dioxide 16, via oxidation of dithiazine 15 (Equation 11) <1997JP11157>. 

An HPLC method was developed for characterisation of the oxidation products. Peracids were found to be more reactive at 40 C than hydrogen peroxide. The peracids were found to give nucleophilic oxidation at high pH and electrophilic oxidation at low pH. Under wash conditions (pH = 10) mainly nucleophilic oxidation was observed. For full implications to the bleach process at wash conditions the heterogenic nature of the last process should also be taken into account... 

Percarboxylic acids. This group of compounds, often simply called peracids, represents the main means used to date of activating H2O2 towards electrophilic oxidations, such as epoxidation, Bayer-Villiger oxidation and heterocyclic A-oxidation. 

The most common method of epoxidation is the reaction of olefins with per-acids. For over twenty years, perbenzoic acid and monoperphthalic acid have been the most frequently used reagents. Recently, m-chloroperbenzoic acid has proved to be an equally efficient reagent which is commercially available (Aldrich Chemicals). The general electrophilic addition mechanism of the peracid-olefin reaction is currently believed to involve either an intra-molecularly bonded spiro species (1) or a 1,3-dipolar adduct of a carbonyl oxide, cf. (2). The electrophilic addition reaction is sensitive to steric effects. 

Double bonds in a,/3-unsaturated keto steroids can be selectively oxidized with alkaline hydrogen peroxide to yield epoxy ketones. In contrast to the electrophilic addition mechanism of peracids, the mechanism of alkaline epoxidation involves nucleophilic attack of hydroperoxide ion on the con-... 

It has been noted63 that in competitive reactions of thiolane-1-oxides, the cis sulphoxide, is oxidized somewhat quicker by peracids than the trans isomer (equation 18). This observation is presumably due to the greater accessibility of the electron pair for electrophilic attack in the cis species. 

Although electrophilic attack by peracids proceeds first at N-3 in simple 1,2,3-thiadiazoles, the sulfur can be oxidized by an excess of reagent to give an A A.A -trioxide <1996CHEC-II(4)289>. 

One can expect that epoxidation occurs as electrophilic reaction. Peracid oxidizes olefin in two forms monomeric and dimeric. The following scheme of epoxidation was proposed [42] ... 

To facilitate electrophilic substitution, it is possible to first convert pyridine into pyridine A-oxide by the action of a peracid such as peracetic... 

In acidic solution, dihydrothiazin-3-ones are in equilibrium with their enol form and susceptible to electrophilic attack at the 2-carbon. They can be oxidized at this site with peracids or diacyl peroxides (Scheme 25) <19828312, 1982S424>. 

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